1. Field of the Invention
This invention relates generally to a trolling motor for a fishing boat. More particularly, but not by way of limitation, the present invention relates to a trolling motor for a fishing boat with an integral sonar transducer for use with a fish locator, depth finder, or the like.
2. Background
Trolling motors are well known in the art as are sonar devices. Generally speaking, a trolling motor is a small electric motor coupled to a propeller for quietly adjusting the position of a fishing boat at relatively low speeds. Trolling motors are available with a variety of features such as variable speed, electric steering, power mounts, etc. Some models connect to a sonar device such as a depth finder to provide the user with navigational options, for example following a straight-ahead course, following a bottom contour, etc.
Sonar devices are also popular equipment for the sport fisherman. Typically, a sonar transducer is placed in the water facing generally downward. A control unit, preferably mounted within view of the fisherman, causes the transducer to emit a pulse of acoustical ultrasonic energy. Upon completion of the pulse, the control unit uses the transducer to “listen” for return echos. By measuring the time period to a particular echo, the control unit can determine the distance between the transducer and an object. By measuring the amplitude of the echo, the control unit may predict the size of the object. Many contemporary control units employ complex computer software and additional sensors to provide information to the fisherman regarding a host of underwater factors and navigational information.
In a conventional installation, an on-board sonar device utilizes a transducer that has been mounted to the hull of the boat. This sort of mounting arrangement, however, is not without its problems. For example, if the transducer is attached to the boat with screws, the screws must necessarily penetrate the outer surface of the hull at a point that may be at least occasionally beneath the water line. Obviously, this can lead to leaks and associated problems. Alternatively, if the transducer is adhesively mounted, the adhesive must be capable of continuous contact with water without becoming compromised. Mounting a transducer to a bracket which is attached to the boat above the waterline can mitigate these concerns to a certain extent, but that solution is not always available.
Since a trolling motor must also be mounted to the boat and since the motor and propeller must be submerged, it would thus appear, at least at first blush, that a trolling motor would provide an ideal platform for placement of an ultrasonic transducer. The prospect of placing a sonar transducer in a trolling motor is even more appealing in light of the fact that, at least with high-end equipment, an ever increasing number of trolling motor features are being coupled to display devices such as sonar control units so that an operator may be provided a visual indication of operating parameters and conditions. Placing a sonar transducer in a trolling motor also simplifies cable routing since the sonar cable may easily be routed through the trolling motor support column.
Unfortunately, prior attempts to locate a sonar transducer in a trolling motor have been hindered by the amount of electrical and electromagnetic noise present in the general vicinity of the trolling motor. Trolling motors generate such noise in at least four areas.
First, by the very nature of the control system that is typically used in combination with a trolling motor. Electronic control of trolling motors and electronic steering systems have almost universally employed pulse width modulation (PWM) schemes, as opposed to linear drive circuitry. PWM control systems operate more efficiently than linear systems which result in components that operate at a substantially lower temperature, utilize less power, and need less complex drive circuitry. However, the general nature of a PWM control system requires production of a substantially rectangular waveform at the output of the driver. Such a waveform inherently contains substantial harmonic content many times over the fundamental frequency of the PWM signal. When a PWM signal is used to drive the motor, the electrical currents are relatively high, thus generating nontrivial amounts of high frequency electrical and electromagnetic noise. If this signal has harmonic content approximately equal to the frequency of the signal produced by the sonar transducer, the received transducer signal may be completely obscured by the noise thus produced. Further, if the transducer cable and the power supply cable for the motor both run through the support column, the opportunity for cross talk from the power cable to the transducer cable is greatly enhanced. Of course, such cross talk poses still another opportunity for the motor drive signal to find its way into the received sonar signal.
Second, noise generally referred to as “brush noise” is created by the motor. As the armature rotates in the motor, the brushes ride on the commutator, cyclically energizing the windings located on the armature. As a particular winding is de-energized during this process, arcing is likely to occur between the brush and the contact on the commutator resulting in electric and electromagnetic noise.
Third, substantial amounts of noise may also be introduced into the received sonar signal through common circuit conductors. Commonly known as ground loops, unwanted electrical currents often flow through conductors when multiple electrical paths are created through the interconnection of multiple circuit elements, e.g., the trolling motor and the sonar control unit. For example, the control unit may be connected to the boat's electrical system, the negative lead of which is normally connected to chassis ground. The trolling motor may be connected instead to a spare battery located in the boat. The negative power supply lead from the spare battery may also be connected to trolling motor chassis components. Once the trolling motor is mounted to the boat, or when placed in the water, an electrical connection is made between the boat electrical system and the trolling motor electrical system. In this case, no currents will flow because there is a single common connection. On the other hand, if, for example, a shield within a signal cable is then connected between the trolling motor and the control unit, unwanted electrical current will likely flow when either an accessories attached on the boat is powered, or when the trolling motor is operational. The resistance of the conductors causes voltage losses when such currents flow which appear as noise to the receiver circuitry. The reactive characteristics of the conductors will likewise induce noise from these currents.
Finally, the spinning armature in the motor produces an external magnetic field which varies over time at a frequency proportional to the rate of rotation of the motor. This varying magnetic field will induce a voltage in nearby conductors. The wires connected to a sonar transducer mounted in a trolling motor will necessarily be subjected to such a magnetic field which will result in additional unwanted noise in the received sonar signal.
The ability of an electronic device to resolve meaningful information from a received signal is determined, in part, by the signal to noise ratio (often given in dB) present at a receiver. Thus, as is well known to those skilled in the art, placing the sonar transducer near the trolling motor will cause the introduction of a substantial degree of noise into the receiver both from noise resulting from the motor drive and possibly from ground loops, thereby reducing the circuit's ability to resolve meaningful information from a received pulse.
Trolling motor models are available which include an integral sonar transducer. However, these trolling motors and the integral sensor only operate with a specific control unit which is properly configured for such operation. These systems are designed such that electrical noise that is created by the drive circuitry for the trolling motor and noise induced from ground loops will appear as common mode noise relative to the transducer output and may be subtracted out of the sonar signal by the receiver circuitry. The circuitry incorporated into the vast majority of trolling motors which are currently available does not allow for this sort of solution. An example of such a combination is disclosed in U.S. Pat. No. 5,525,081 issued to Mardesich, et al. which is hereby incorporated by reference.
It is thus an object of the present invention to provide a trolling motor with integral sonar transducer which will operate with an existing sonar control unit. It is also a related object of the present invention to provide a trolling motor with integral sonar transducer wherein there is substantial improvement in the signal to noise ratio of the signal presented to the sonar receiver from the transducer so incorporated into the trolling motor.
It is a further object of the present invention to provide a cable for connection between a sonar transducer and a sonar control unit which provides improved shielding from sources of high frequency noise in close proximity to the cable.
It is yet another object of the present invention to provide a method for electrically connecting a trolling motor with integral sonar transducer to a power source and to a sonar control unit so as to eliminate multiple ground paths on the sonar signal or reduce the effect thereof.